Magnetic resonance imaging methods and apparatus for time-series motion tracking with inversion recovery compensation

Abstract

Image contributions produced by an untagged specimen magnetization component in magnetic resonance imaging are controlled by applying one or more radiofrequency (RF) pulses that invert at least a portion of the untagged specimen magnetization. In an example, a specimen is tagged with a spatially modulated magnetization that is used to produce an image signal that includes a contribution associated with the tagged magnetization and an untagged magnetization. The untagged magnetization is substantially along an axial direction defined by an applied axial magnetic field. The untagged magnetization increases in magnitude because of so-called T 1 relaxation. A contribution to the image signal increases for a predetermined time or to a predetermined magnitude, and a 180-degree pulse is applied to invert at least a portion of the untagged magnetization. The untagged magnetization is then antiparallel with respect to the applied axial magnetic field. Additional inversion recovery causes the untagged magnetization to increase from a negative value to zero and then becomes positive. As a result, signal contributions associated with the untagged magnetization are reduced. Additional 180-degree pulses can be applied whenever the untagged magnetization becomes larger than a predetermined value so that image contrast can be maintained. When the tagged magnetization decreases to a predetermined level, an initial specimen magnetization is reestablished for subsequent imaging.

1. A magnetic resonance imaging method, comprising:
 establishing a tagged magnetization simultaneously throughout a specimen; 
 repetitively applying a compensation pulse to the specimen, wherein the compensation pulse is configured to invert at least an untagged specimen axial magnetization component associated with decay of the tagged magnetization; 
 applying an imaging pulse sequence after each repetitively applied compensation pulse; and 
 obtaining images of at least a portion of a specimen based on image signals associated with the imaging pulse sequences, wherein the image signals include contributions from a tagged axial magnetization component and an untagged specimen magnetization associated with the decay of the tagged magnetization. 
 
     
     
       2. The method of  claim 1 , wherein the compensation pulse is substantially a π-pulse. 
     
     
       3. The method of  claim 1 , wherein the image signal is based on a spatially varying amplitude of the tagged axial magnetization component. 
     
     
       4. The method of  claim 1 , wherein the image signal is based on a spatially varying phase of the tagged axial magnetization component. 
     
     
       5. A method for magnetic resonance imaging, comprising:
 establishing an initial spatially tagged magnetization throughout a specimen volume; 
 repetitively obtaining image signals associated with the initial spatially tagged magnetization in the volume, wherein image signal acquisitions are interleaved with applications of a compensation pulse that is selected to invert at least a portion of the spatially tagged magnetization and an untagged magnetization produced by decay of the spatially tagged magnetization. 
 
     
     
       6. The method of  claim 5 , further comprising:
 establishing a subsequent spatially tagged magnetization simultaneously throughout a specimen volume; 
 repetitively obtaining image signals associated with the subsequent spatially tagged magnetization in the volume, wherein image signal acquisitions are interleaved with applications of a compensation pulse that is selected to invert at least a portion of the spatially tagged magnetization and at least a portion of an untagged magnetization produced by decay of the spatially tagged magnetization. 
 
     
     
       7. The method of  claim 6 , wherein the subsequent spatially tagged magnetization is re-established after a time interval associated with a decrease in magnitude of the initial spatially tagged magnetization. 
     
     
       8. The method of  claim 6 , further comprising displaying images based on the image signals. 
     
     
       9. A method, comprising:
 (a) situating a specimen in a substantially constant magnetic field that is directed along a z-axis so as to establish a substantially constant magnetization in at least a selected portion of the specimen, the constant magnetization having a magnitude M 0 ; 
 (b) selecting a flip angle α; 
 (c) establishing a stored longitudinal magnetization having an unencoded component with an initial magnitude M U  and an encoded component with an initial magnitude M ENC  in a specimen, wherein the magnitudes M U  and M ENC  are based on the selected flip angle α and are proportional to the magnitude M 0  of the constant magnetization; 
 (d) allowing the stored longitudinal magnetization to evolve for a time τ 1  such that the magnitude of the encoded component of magnetization is proportional to M ENC  exp[−τ 1 /T 1 ] and the magnitude of the unencoded component of magnetization is based on a sum of M 0 (1−exp[−τ 1 /T 1 ]) and M U  exp[−τ 1 /T 1 ]; 
 (e) obtaining at least part of a specimen image based on the evolved stored longitudinal magnetization; 
 (f) inverting the evolved stored longitudinal magnetization and allowing the inverted magnetization to evolve for a time τ 2  such that the magnitude of the encoded component of magnetization is substantially proportional to −M ENC  exp[−(τ 1 +τ 2 )/T 1 ] and the unencoded component is substantially proportional to −M U  exp[−(τ 1 +τ 2 )/T 1 ]; and 
 (g) obtaining at least part of an image based on the inverted evolved magnetization. 
 
     
     
       10. The method of  claim 9 , wherein the flip angle α is selected so that the initial magnitudes M U  and M ENC  are substantially the same. 
     
     
       11. The method of  claim 9 , wherein the flip angle is selected a so that the initial magnitude M U  is substantially zero. 
     
     
       12. The method of  claim 9 , wherein τ 1  and τ 2  are substantially equal. 
     
     
       13. The method of  claim 12 , further comprising repeating steps (d)-(g) to obtain a plurality of specimen images.